Prior to development of self-converging yokes, beam convergence was usually achieved by use of dynamically varied magnetic fields that were coupled to plates or pole pieces located at the output end of an electron gun. The magnetic fields were formed by electromagnetic components located outside the neck of the tube. However, the adjustments for such a dynamic convergence system was extremely complex and time consuming. In response to this adjustment problem, a system utilizing a self-converging yoke was developed.
Although most present-day deflection yokes produce a self-convergence of the three beams in a cathode-ray tube, the price paid for such self-convergence is a deterioration of the individual electron beam spot shapes. The self-converging yoke magnetic field is astigmatic. It both overfocuses the vertical-plane electron beam rays, leading to deflected spots with appreciable vertical flare, and underfocuses the horizontal-plane rays, leading to slightly enlarged spot width.
It is desirable to avoid the astigmatism problem associated with a self-converging yoke by use of a yoke that is not self-converging. However, it is not desirable to return to use of dynamically varied magnetic fields for converging the beams.
The present invention provides a system that uses both a non-converging yoke and an electron gun that includes means for converging the electron beams.